The present invention relates to improvements in the art of plastic film shrink overwrap packaging.
Various polymer acronyms are used herein and listed below. A colon (:) will be used to indicate components to the left and right of the colon are blended. A slash "/" will be used to indicate that components to the left and right of the slash are in different layers and the relative position of layers. Acronyms include:
PE-Polyethylene (an ethylene homopolymer and/or copolymer of a major portion of ethylene with one or more .alpha.-olefins) PA1 HDPE-High Density Polyethylene, A homopolymer of ethylene or a copolymer of a major portion by weight of ethylene with one or more .alpha.-olefins, the homopolymer or copolymer having a density greater than 0.940 g/cm.sup.3, and a melting point between 125.degree. C. and 140.degree. C. PA1 LDPE-Low Density Polyethylene, A highly branched homopolymer of ethylene having a density between 0.915 and 0.930 g/cm.sup.3, typically having long branches off the main backbone chain with alkyl substituents of 2 to 8 carbon atoms on these branches. PA1 VLDPE-Very Low Density Polyethylene, A copolymer of a major portion by weight of ethylene with one or more .alpha.-olefins, the copolymer having a density greater than 0.86 and less than 0.915 g/cm.sup.3, a melting point between 85.degree. C. and 125.degree. C., and a Vicat Softening Point (VSP) greater than 60.degree. C. PA1 LLDPE-Linear Low Density Polyethylene, A copolymer of a major portion by weight of ethylene with one or more .alpha.-olefins, the copolymer having a density of at least 0.915 g/cm.sup.3, a melting point between 115.degree. C. and 130.degree. C., and a Vicat Softening Point (VSP) greater than 60.degree. C.
Various published documents disclose different types of packaging films.
U.S. Pat. No. 4,551,380 (Schoenberg) discloses an oriented, heat sealable, multilayer packaging film having a structure of LLDPE:LMDPE:EVA/LLDPE/LLDPE:LMDPE:EVA, where the film is crosslinked and heat shrinkable. Use of erucamide slip agents and diatomaceous earth antiblock agents are also disclosed.
U.S. Pat. No. 4,760,116 (Roberts) discloses a heat shrinkable film which is crosslinked at an irradiation dosage of from 1 to 5 megarads and is a blend of linear low or medium density polyethylene with a copolymer of ethylene and a vinyl acetate, acrylic acid, or ester, such as EVA. Use of erucamide slip agents and silica antiblock agents is also disclosed.
U.S. Pat. No. 5,707,751 (Garza et al) discloses multilayer shrink films of which at least one layer is a blend of a metallocene catalyzed polyolefin having a density &lt;0.92 g/cm.sup.3 and a melting point &lt;115.degree. C. with a polyolefin having a melting point that is at least 10.degree. C. greater than the first mentioned polyolefin. It is further stated that this blend may also be further blended with "one or more additional polymers or copolymers such as other VLDPEs, LDPE, HDPE, LLDPE, polypropylene, propylene copolymers, styrene polymers and copolymers, polyester, nylon, PVDC, EVOH, ethylene-vinylacetate copolymer (EVA) and ionomers."
Various monolayer and multilayer thermoplastic heat shrinkable films have been commercialized for overwrap packaging of e.g. food such as poultry and cheese, industrial products such as tools and building components, and consumer products such as toys, tapes, and reading materials. One to six layer films are common. Typical structures include: LLDPE:EVA and LLDPE:EVA/LLDPE/LLDPE:EVA/LLDPE/LLDPE:EVA. Some are irradiatively crosslinked and/or corona treated or not.
Also known are films suitable for packaging foodstuffs that are heat shrinkable at temperatures from 80.degree. C. to 127.degree. C. and which are heat sealable using well known commercially available sealers.
These films are typically provided to end users for packaging goods as sheets wound on rolls which are often center folded. In a typical packaging operation, the folded film passes over a separator bar with opposing film sides on either side of the bar. The lower film side is passed beneath a support plate on which goods to be packaged are placed. The goods and film are then transferred to a sealing station where an L-bar seal is applied which simultaneously seals and cuts the folded film. The film trim is continuously removed and wound on a reel as each package advances through the sealing operation. A subsequent L-bar seal provides the final leg of a U-shaped seal around the goods and with the fold provides a sealed envelope around the goods. The sealed package is then transferred to a hot air oven or tunnel where the film is heat shrunk around the goods. The seal may be hermetic thereby providing a balloon package for cushioning, but often the film is perforated prior to sealing and/or heat shrinking so that the film shrinks tight around the goods.
Generally heat sealing of thermoplastic film is accomplished by applying sufficient heat and pressure to adjacent film layer surfaces for a sufficient time to cause a fusion bond between the layers.
A common type of seal used in manufacturing overwrapped packaged goods is known to those skilled in the art as an impulse seal. An impulse seal is made by application of heat and pressure using opposing surfaces at least one of which is a heating surface such as a wire, knife, covered wire or ribbon through which electric current is passed for a very brief time period (hence the name "impulse") to generate heat which thereby causes the adjacent film layers to fusion bond. An impulse sealer using a wire may also use the wire to cut through the film layers to provide separate packages. Typically, the surface opposing the heated surface is a resilient, heat resistant pad.
Hot bar seals may also be used. In making a hot bar seal, adjacent thermoplastic layers are held together by opposing bars of which at least one is heated to cause the adjacent thermoplastic layers to fusion bond by application of heat and pressure across the area to be sealed. The impulse seal or hot bar,seal may be used in parallel pairs with a cutting knife located between the seals.
One problem which occurs during the packaging operation is that the film sticks to the separator bar causing the film to jam or back up at that point interfering with production. Another problem is puckering of the film along the sealed edge. Yet another problem is stickiness of the film after packaging as it slides against adjacent similarly packaged goods during packing into a carton. Such stickiness can make it difficult e.g. to package plastic film coated cartons into a box and in particularly packing the last carton into the box may be very difficult unless the film will slide easily against itself. When this sliding occurs immediately after the package comes from the shrinking step which performed at elevated temperatures, sticking is a problem. The ability of the hot film to slide against itself after packaging is known in the art as "hot slip".
Irradiation of a multilayer film causes the various irradiated layers in the film to crosslink. Under controlled conditions, crosslinking by irradiation raises and may also broaden the temperature range for heat sealing.
Disadvantageously, if the heat sealing layer of the thermoplastic film is crosslinked too heavily, the highly crosslinked layer is more difficult to melt or fusion bond which makes it difficult to achieve strong seals, particularly by impulse sealing. All of the seals should maintain their integrity to protect the enclosed products and provide a good appearance.
Some applications require a strong continuous seal to prevent unwanted egress and ingress of gaseous, liquid or solid materials between the bag exterior and interior. This is particularly necessary for food-containing packages.
There is a continuing need for multilayer films which can be used to package a variety of goods providing strong seals especially when formed by impulse sealing and having good slip characteristics and good optical properties. Such films should be able to produce such seals over a wide sealing temperature range in a film having a combination of good optical properties, tear resistance, high shrinkage values, and good slip characteristics.
Variations in sealing temperatures, times and pressure are known to exist not only from one brand and/or type of sealers to another but also between different sealing machines sold by the same manufacturer under the same brand identification. Such variations, which may be due to factors such as variation in the manufacturer's product or varying equipment settings or installation, increase the desirability for films which may be heat sealed to produce strong integral seals over a wide range of temperatures and therefore be usefully sealed on different sealing machines.
Another problem encountered during heat sealing is that of inadvertent folding. Normally, a heat seal is made by applying heat and pressure across two sheets or portions of film e.g. the two opposing sides of a flattened tube, however, occasionally the area to be sealed will be inadvertently folded to produce a section of film having four or six sheets or film portions which are pressed between the opposing sealer bars. A wider impulse heat sealing temperature range is indicative of a greater latitude in sealing through folds than a narrower range.